As others have mentioned, it refers to the gear shift. But it actually has a meaningful origin - many years ago, the gear order was not agreed upon. Many cars had a gear shift that was PNDLR (which I’ve heard pronounced “pendler”), where reverse was at the end. At the time, it was useful to tell the difference between a PRNDL and a PNDLR shift.

Of course that was all before 1971, when PRNDL was mandated by the US government.